1. Technical Field
The present invention relates generally to the production of oil from a subterranean oil-bearing formation and, more particularly, to a method for determining an optimum gas injection rate for a gas-lifted oil well penetrating the formation.
2. Background Information
Gas injection into an oil well is a well-known artificial lift method that facilitates oil recovery from the well. A typical gas lift method provides a lift gas at the surface that is conveyed through a gas flow control valve to a surface wellhead connection where the lift gas is injected into the casing-tubing annulus of the well. The adjustable position of the gas flow control valve regulates the gas injection rate into the well. Upon injection, the lift gas travels down the casing-tubing annulus to a plurality of specially designed subsurface gas injection valves that enable the lift gas to enter the tubing string. The lift gas commingles with the reservoir fluids in the tubing string, lifting the reservoir fluids up the tubing string to the surface for the recovery of oil therefrom. The injection rate of the lift gas into the well is an important parameter for the economics of the well because there are significant operational costs associated with gas injection. Accordingly, it is desirable to economically optimize the gas injection rate into the well such that the operational costs for the well are balanced with the oil production revenue from the well.
The concept of slope is associated with the optimum gas injection rate for a gas-lifted oil well, wherein the gas-lift slope is defined as the incremental change in the liquid production rate divided by the incremental change in the gas injection rate for the well at any point on a plot of the gas injection rate versus the liquid production rate. Such plots are theoretically developed from published empirical correlations. The theoretical optimum gas injection rate for a gas-lifted oil well generally corresponds to the optimum gas-lift slope which can be derived from conventional equations known in the prior art. Thus, it is possible to mathematically determine the theoretical optimum gas injection rate for a gas-lifted oil well using empirical correlations. However, the actual or field-measured optimum gas injection rate for a gas-lifted oil well must be determined in the field by direct testing of the oil well for liquid productions rates at various gas injection rates. An ongoing need exists for improved means of determining the actual optimum gas injection rate for a gas-lifted oil well.
Accordingly, it is an object of the present invention to provide an effective method for determining the actual optimum gas injection rate for a gas-lifted oil well. It is also an object of the present invention to provide such a method that determines the actual optimum gas injection rate automatically once the method is initiated. It is another object of the present invention to provide such a method that determines the actual optimum gas injection rate from measured field data including liquid production rates and gas injection rates, rather than empirical correlations. It is still another object of the present invention to provide such a method that determines the actual optimum gas injection rate by automatically and iteratively adjusting the actual gas injection rate into the well at the surface via a gas flow control valve. It is a further object of the present invention to provide such a method that continuously injects a lift gas into the gas-lifted oil well at the optimum gas injection rate for continuous oil production upon completion of the method by automatically fixing the position of the surface gas flow control valve. It is a further object of the present invention to provide such a method the optimum gas injection rate is based on economic criteria. These objects and others are achieved in accordance with the invention described hereafter.